1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a chip on glass (COG) liquid crystal display (LCD) device.
2. Discussion of the Related Art
Among the various flat display devices, liquid crystal display (LCD) devices have been widely used due to their advantageous characteristics such as high contrast ratio, good gray scale, good moving picture quality, and low power consumption.
Generally, the LCD device includes a color filter CF array substrate having a color filter layer to represent colors, a thin film transistor TFT array substrate being disposed opposite to the CF array substrate, a liquid crystal layer formed between the TFT array substrate and the CF array substrate, and a driving unit for driving the TFT array substrate.
The TFT array substrate includes a plurality of gate and data lines crossing each other to define pixel regions, a plurality of thin film transistors TFT formed adjacent to the crossings of the gate and data lines, and storage capacitors for maintaining a charged state of the pixel region until the region is addressed next time.
The driving unit includes a gate driver for driving the gate lines, a data driver for driving the data lines, a timing controller for controlling the gate and data drivers, and a power supply for providing driving voltages used for driving the LCD device. The timing controller controls driving timings of the gate and data drivers, and provides a video signal to the data driver. Also, the power supply generates the driving voltages including a common voltage Vcom, a gate high voltage Vgh and a gate low voltage Vgl from an input power, to thereby drive the LCD device.
The gate driver sequentially provides a scanning signal to the gate lines to drive the pixel regions a line at a time. Also, the data driver provides a video signal to the respective data lines whenever the scanning signal is provided to any one of the gate lines. Accordingly, the LCD device can control the light transmittance by an electric field generated between a pixel electrode and a common electrode, to thereby display images.
Each of the gate and data drivers is provided with a plurality of drive integrated circuit (IC) chips. A process for providing the drive IC chips to the gate and data drivers is classified into a tape automated bonding (TAB) method and a chip on glass (COG) method. In the TAB method, the gate and data drive IC chips are mounted on a tape carrier package (TCP), and then are brought in contact with an LCD panel. In the COG method, the drive IC chips are directly mounted on the TFT array substrate. Recently, so as to obtain a thin profile of the LCD device by minimizing the size of PCB, a trend has been to use a line on glass (LOG) method in which signal lines being in contact with the drive IC chips are formed on a glass substrate.
Hereinafter, a TFT array substrate of an LCD device according to a related art will be described with reference to the accompanying drawings.
FIG. 1 is a plan view illustrating a COG type LCD device according to a related art. FIG. 2 is a schematic view illustrating an arrangement of the data drive IC chips according to the related art.
As shown in FIG. 1, a TFT array substrate 150 is divided into an image display area 152 and a non-image display area 154.
On the image display area 152 of the TFT array substrate 150, a plurality of data lines 162, a plurality of gate lines 161, a plurality of thin film transistors TFT, and a plurality of pixel electrodes 110 are formed. Each data line 162 transmits a video signal. The gate lines 161 are formed perpendicular to the data line 162 to define a plurality of unit pixel regions. The gate lines 161 transmit a scanning signal to each pixel region. The thin film transistors TFT are formed adjacent to the crossings of the gate and data lines 161 and 162. The thin film transistors TFT each includes a gate electrode, a gate insulating layer, a semiconductor layer, and source and drain electrodes. Each pixel electrode is connected with the drain electrode of a corresponding thin film transistor TFT by interposing a passivation layer therebetween. 013 On the non-image display area 154 of the TFT array substrate 150, respective ends of the gate and data lines 161 and 162 are connected with a plurality of gate and data drive IC chips 170 and 180 adhered to the TFT array substrate 150.
The gate drive IC chips 170 are connected to one another by an LOG line 182 formed on the TFT array substrate. Also, the data drive IC chips 180 are connected to one another by another LOG line 182. The LOG lines 182 are connected to a printed circuit board (PCB) for providing various signals via a cable 190. That is, the various signals from the PCB are transmitted to the gate and data drive IC chips 170 and 180 by the LOG line 182.
To minimize the line connections, the data and gate drive IC chips 170 and 180 are cascaded on the TFT array substrate 150 by the LOG lines 182. Then, a power signal, a video interface signal and a sampling start pulse (SSP) signal are inputted to the drive IC chips. However, when the drive IC chips are cascaded, a problem occurs in that the driving voltage outputted from the data drive IC chips may drop.
That is, as shown in FIG. 2, even though the PCB provides a constant driving voltage Vin to the data drive IC chips through the cable 190, the driving voltage drops going from the PCB due to the resistance of the PCB and the LOG line 182. For example, if the driving voltage Vin is 5.0V, a voltage V1 inputted to the first data drive IC chip (D-IC(1)) may drop to 4.96V, a voltage V2 inputted to the second data drive IC chip (D-IC(2)) may drop to 4.50V, and a voltage V3 inputted to the third data drive IC chip (D-IC(3)) may drop to 4.00V.
The voltage inputted to the respective data drive IC chips changes due to the resistance of the LOG line 182. Accordingly, in a black image (in case of a normally white mode) having a high current, the data drive IC chips output different driving voltages, whereby the pixel voltages applied to the respective data lines are also changed. As a result, it is difficult to align liquid crystal molecules to a desired direction, thereby deteriorating the brightness of the image.